The present invention relates to a radiotelephone operating in a time-divided radiotelephone system, which telephone has at least a first mode and a second mode, whereby in the first mode the transmitter is active and in the second mode the receiver is active.
In TDMA, i.e. time-divided radiotelephone systems, the transmitter and receiver are on at different times. Accordingly, connecting the transmitter and receiver to the antenna can be done either with synchronized switches (PIN diode, GaAs-fet switch, or the like) or with a duplex filter that is familiar from analog telephone applications. A typical block diagram using a duplex filter is shown in FIG. 1.
To reduce power consumption and block leakage of the TX-frequency signal to the RX branch, the receiver's electric power (VRX) is switched off when the transmitter (TX) is on. In this case, at the antenna port of the duplex filter there is a signal level that is of the order of +30 dBm at its greatest, in other words, when the transmitter is using the highest power level.
The isolation from the transmitter to the input of the receiver may be less than 20 dB at its poorest (1800 MHz, i.e. in the PCN range), in which case during transmission a still quite strong signal (&gt;10 dBm) is connected to the receiver (RXamp). Although the receiver stage does not carry a supply voltage during transmission, the transmitter signal leaks through it. In addition, the signal is not attenuated very much in the RX filter (RXfil), because the highest transmission frequency is only 20 MHz away from the lowest reception frequency and the reception band is all of 75 MHz wide. When the transmission signal reaches the RX mixer with a sufficient intensity, it causes a frequency pulling of the local oscillator (VCO), which shows up in the transmitted signal as a phase error because the same oscillator is used to form the transmission frequency. To minimize this effect, buffering can be added in between the oscillator and the RX mixer or the isolation from the transmitter to the receiver can be improved. Improving the isolation by tightening the duplex filter nevertheless increases the price of the filter and the attenuation on the upper channels of the transmitter.
The above-described leakage problem also hampers systems operating in a lower frequency than PCN range, though it is somewhat less pronounced.
A well known means of remedying the problem discussed is thus to use a better duplex filter whose RX branch attenuates the TX frequency signal more. This nevertheless complicates the structure of the filter, and makes it bigger and more costly. Another problem is that now attenuation of the duplex filter for TX frequency signals increases, in this case on the upper channels. Thus, the transmitter power going to the antenna tends to diminish on the upper channels, and it has to be compensated by increasing the power obtainable from the transmitter, which on the other hand increases the current consumption.
Another well known way is to add a buffer amplifier in between the RX mixer and the VCO. This reduces the leakage from the RX mixer to the VCO. In order for the buffer amplifier to work in the desired manner, it must of course be active, and this increases current consumption.
Some telephone manufacturers use in place of a duplex filter a changeover switch which serves to select the signal path TX-amp . . . &gt;ANT during transmission and ANT . . . &gt;RX-amp during reception. These switches too cause signal leakage at high frequencies.
The leakage of a quite strong transmission signal into the telephone's other blocks causes problems similar to those in a VCO. A problem point is, for example, at least the integrated circuits, where it is attempted to integrate both the receiver and transmitter blocks on the same circuit.